Positioning in a telcommunications system

ABSTRACT

A telecommunications system comprises a first transmitter unit situated at a first, known location; a second transmitter unit situated at a second, unknown location; a first receiving unit at a third, known location arranged to receive signals from the first and second transmitter units; and a second receiving unit at a fourth, known location arranged to receive signals from the first and second transmitter units, wherein the said signals received by the first and second receiving units are usable to ascertain the location of the second transmitter unit.

FIELD OF INVENTION

The present invention relates to a telecommunications system and amethod of determining the location of a transmitter unit in thetelecommunications system.

BACKGROUND OF INVENTION

The ability to pinpoint the location of mobile telephones is a desirablefeature of a mobile telephone network. This is because of the need toprovide customer services which rely on knowing the whereabouts of usersof these services. For example, up-to-date local traffic information canbe provided to enable a user to avoid nearby traffic jams. A user mayalso wish to know, for example, how to get to the nearest pub orrestaurant from their present location. Clearly the location of the usermust be ascertained to within even a few metres for this type of serviceto work.

Another reason for wishing to know the location of a mobile telephone isso that emergency services can locate a caller who is unable to providean accurate personal location themselves.

The ability to pinpoint the location of a mobile telephone normallyrelies on ascertaining its position relative to known positions ofmobile network entities such as base stations. It is known in the artand has been observed by the present inventors during mobile stationlocation trials, that it can be very difficult to obtain accurateinformation about mobile telephone networks. Examples of problematicinformation include base station coordinates, base station identitiesand transmission channels used. This type of data is provided by thenetwork operator and it is quite common for such data to be incorrect,for example because it has not been updated.

One specific problem is incorrect base station coordinates. If it isintended to pinpoint the location of a mobile telephone relative to abase station, it is clear that any location of the mobile telephonecalculated on the basis of the base station coordinates will also beincorrect. The present inventors have noticed errors of up to severalhundreds of meters in different countries in base station positions.Such large errors clearly have a negative impact when providing the typeof customer services described above.

Another specific problem arises when base stations are identifiedwrongly. For example, in a GSM network a base station can be identifiedby its Broadcast Control Channel (BCCH) frequency and Base StationIdentity Code (BSIC) value. This information is also provided by thenetwork operator and if it is not correct, the wrong base station isidentified, and its coordinates are used, which in turn means that thewrong coordinates are used for the calculation of mobile telephonelocation. Clearly, even with a relatively large number of base stationsin a given network area, using the wrong coordinates could have asignificant impact on the calculated mobile telephone position and hencethe ability to advise the user of local information.

Yet another specific problem is that, depending on the particularoperator, the transmission channels used might be changed relativelyoften. If non-updated channel information is used for mobile stationlocation calculation, when frequencies have been changed, again, basestations are incorrectly identified.

It is also useful to know base station co-ordinates for the purposes ofnetwork planning to allow normal call and data operation. Even thoughthe accuracy required for this is less than that required for theabove-described location services, errors of several hundred meter canalso cause problems in network planning.]

It would be desirable to provide a method of checking the consistency ofnetwork information such as the operator-provided information describedabove. Ideally such a method would be easily implementable in networkssupporting E-OTD location method in GSM, and OTDOA-IPDL method in UMTS.

SUMMARY OF INVENTION

According to a first aspect of the present invention there is provided atelecommunications system comprising: a first transmitter unit situatedat a first, known location; a second transmitter unit situated at asecond, unknown location; a first receiving unit at a third, knownlocation arranged to receive signals from the first and secondtransmitter units; and a second receiving unit at a fourth, knownlocation arranged to receive signals from the first and secondtransmitter units, wherein the said signals received by the first andsecond receiving units are usable to ascertain the location of thesecond transmitter unit.

Preferably the signals are indicative of the time taken for the signalsto arrive at the first and second receiving units from the first andsecond transmitters. The signals can be used to determine the timedifference between the arrival times of signals at the first and secondreceiving units from the first and second transmitters.

Conveniently the first and/or second receiving units are moveablebetween a plurality of locations and are both arranged to receive a pairof signals when in each of the plurality of locations, the said pair ofsignals comprising a signal from the first transmitter unit and a signalfrom the second transmitter unit. In this case, a said pair of signalsreceived by the first receiving unit and a said pair of signals receivedby the second receiving unit can be used together to calculate a rangeof possible locations of the second transmitter unit. The range ofpossible locations is usually in the form of a hyperbola in the X-Yplane in which the second transmitter unit is located, the saidhyperbola running through substantially the location of the secondtransmitter unit.

In each of the plurality of locations the first and second receivingunits receive pairs of signals which can differ from those pairs ofsignals received when the first and second receiving units are in othersof the plurality of locations and the said different pairs of signalsare together usable to calculate different ranges of possible locationsof the second transmitter unit. In this case the different ranges ofpossible locations substantially coincide at a single common locationthat is substantially the location of the second transmitter unit. Inany given location of the first and second receiving units, the pair ofsignals received by the first receiving Unit could be the same pair ofsignals that is received by the second receiving unit.

Alternatively, in any given location of the first and second receivingunits, the pair of signals received by the first receiving unit could bea different pair of signals from the pair of signals received by thesecond receiving unit. For example, during one moment in time the firstand second receivers could each measure a different pair of signals andat the next moment they could each measure the pair that the othermeasured earlier.

In either case, conveniently the plurality of locations is threelocations.

It is possible for the signals received by the first and secondreceiving units to be received in response to signals sent to the firstand second transmitter units by the first and second receiving units.

The said signals received by the first and second receiving units mayfurther indicate their quality or accuracy.

The first and second receivers can be separate entities or the sameentity. In the latter case, the said same receiver entity could bearranged to act as the said first receiver during a first period of timeand as the said second receiver during a second separate period of time.

Conveniently, one or both of the first and second receivers is a mobiletelephone. Advantageously, such a mobile telephone supports EnhancedObserved Time Difference (E-OTD) location method and Global PositioningSystem (GPS) location method, or Observed Time Difference Of Arrival(OTDOA) location method and Global Positioning System (GPS) locationmethod.

Preferably, one or both of the first and second transmitter units is acellular base station.

The first or second receivers could be location measurement units.

Usually, the second transmitter unit is in a fixed location.

Preferably the telecommunications system, further comprises acalculation unit arranged to use the signals received by the first andsecond receiving units or any values derived from the said signals toascertain the location of the second transmitter unit. The calculationunit can be arranged to take account of the indication of quality oraccuracy when using the signals received by the first and secondreceiving units.

If the telecommunications system is located within a telecommunicationsnetwork, the calculation unit could be a network management unit or aServing Mobile Location Centre.

The calculation unit can be arranged to verify the accuracy of theascertained location of the second transmitter unit by comparing it withlocation information of the second transmitter unit obtained from othersources. The ascertained location of the second transmitter unit couldalso be used to check the accuracy of identification information of thesecond transmitter unit obtained form other sources and thus identifythe second transmitter.

According to a second aspect of the invention, there is provided atelecommunications system comprising: a first transmitter unit situatedat a first, known location; a second transmitter unit situated at asecond, fixed, unknown location; a first receiving unit at a third,known location arranged to receive signals from the first and secondtransmitter units: and a second receiving unit at a fourth, knownlocation arranged to receive signals from the first and secondtransmitter units, wherein the said signals received by the first andsecond receiving units are usable to ascertain the location of thesecond transmitter unit.

According to a third aspect of the invention, there is provided atelecommunications system comprising: a first base station situated at afirst, known location; a second base station situated at a second,unknown location; a first mobile station at a third, known locationarranged to receive signals from the first and second base stations; anda second mobile station at a fourth, known location arranged to receivesignals from the first and second base stations, wherein the saidsignals received by the first and second mobile stations are usable toascertain the location of the second base station.

According to a fourth aspect of the invention, there is provided amethod of determining the location of a transmitter unit in atelecommunications system, the method comprising the steps of: receivingsignals at a first receiving unit situated at a first, known locationfrom a first transmitter unit situated at a second, known location andfrom a second transmitter unit situated at a third, unknown location;receiving signals at a second receiving unit situated at a fourth, knownlocation from the said first transmitter unit and from the said secondtransmitter unit; and using the received signals to ascertain thelocation of the second transmitter unit.

According to a fifth aspect of the invention, there is provided a methodof determining the location of a transmitter unit in atelecommunications system, the method comprising the steps of receivingsignals at a first receiving unit situated at a first, known locationfrom a first transmitter unit situated at a second, known location andfrom a second transmitter unit situated at a third, fixed, unknownlocation; receiving signals at a second receiving unit situated at afourth, known location from the said first transmitter unit and from thesaid second transmitter unit; and using the received signals toascertain the location of the second transmitter unit.

According to a sixth aspect of the invention, there is provided a methodof determining the location of a base station in a telecommunicationssystem, the method comprising the steps of: receiving signals at a firstmobile station situated at a first, known location from a first basestation situated at a second, known location and from a second basestation situated at a third, unknown location; receiving signals at asecond mobile station situated at a fourth, known location from the saidfirst base station and from the said second base station; and using thereceived signals to ascertain the location of the second base station.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying drawings in which:

FIG. 1 shows in general terms a part of a mobile telephone network;

FIG. 2 shows the part of the network of FIG. 1 with some of the networkentities in more than one location;

FIG. 3 shows the part of the network of FIG. 1 with the network entitiesmore specifically identified, in accordance with a first example of theinvention;

FIG. 4 shows the part of the network of FIG. 1 with the network entitiesmore specifically identified, in accordance with a second example of theinvention; and

FIG. 5 shows the part of the network of FIG. 1 with the network entitiesmore specifically identified, in accordance with a third example of theinvention.

In the figures like reference numerals indicate like parts and lowercase letters are used to distinguish the same component when indifferent locations.

DESCRIPTION OF PREFERRED EMBODIMENTS

In embodiments of the invention a moving measurement unit collectsinformation about the network to which it is connected and then usesthis collected data to calculate any missing information about networkentities. That information in turn can be used to pinpoint the locationof mobile telephones or other entities which are using the network. Oneparticular embodiment described below is directed towards calculatingthe position of a transmitter.

Referring firstly to FIG. 1, there are shown, in general terms, fourentities of a mobile telephone network. The first of these is a knowntransmitter (KT) 1, which is at a known location within the network. Itmay be a base station or other transmitter. The second is an unknowntransmitter (UT) 2, which is at an unknown location within the network.It is the location of UT 2 that the embodiment aims to pinpoint. Thethird entity is a first receiver (FR) 4 and the fourth is a secondreceiver (SR) 6. At least one of the FR 4 and the SR 6 is moveablewithin the network and their positions are always known.

In order to ascertain the location of the UT 2, the following procedureis carried out:

(i) The FR 4 performs an E-OTD (Estimated Observed Time Difference)measurement. This is hereinafter referred to as OTD_(FR). The OTD is theobserved time difference between a received signal from the KT 1 and areceived signal from the UT 2, or in other words the elapsed timebetween the arrivals of signals from the two base stations at the firstreceiver.

In a GSM cellular system, the OTD is the time measured between thereceptions of beginnings of bursts from the KT 1 and the UT 2 and it canbe measured e.g. in nano or micro seconds or GSM bit period. Normalbursts, synchronisation bursts and dummy bursts are examples of the typeof signal bursts that can be used. Since GSM uses periodic transmissions(one burst is 0.577 ms long), in practice the FR 4 can measure first thearrival of the signal from one of the KT 1 and the UT 2, and then thearrival of the signal from the other.

In either a GSM or a 3G system, the usual signals transmitted by the KT1 and the UT 2 can be used for the OTD measurement. Alternatively itwould be possible for the FR 4 to send out a signal to the KT 1 and theUT 2 and measure a signal received in response.OTD _(FR) =RTD+GTD _(FR)   (1)where RTD is the Real Time Difference between the KT 1 and the UT 2,i.e. actual time difference between the transmissions of the two signalsfrom the KT 1 and the UT 2. This can alternatively be referred to as theactual transmission time difference. GTD_(FR) is the Geometric TimeDifference for the first receiver, which is the time difference betweenthe propagation times of signals from the UT 2 and KT 1 to the FR 4,i.e. the time difference between the times taken for signals to travelthe geometric distance from the transmitters KT 1 and UT 2 to the FR 4.Both RTD and GTD are suitably given in time units of nano or microseconds.

Thus GTD_(FR) is given by the equation:GTD _(FR) =[d(FR,UT)−d(FR,KT)]/c   (2)where d(FR,UT) is the distance between the FR 4 and the UT 2, d(FR,KT)is the distance between the FR 4 and the KT 1 and c is the speed ofradio waves.

(ii) The SR 6 performs an E-OTD measurement in a similar manner, whichis hereinafter referred to as OTD_(SR).OTD _(SR) =RTD+GTD _(SR)   (3)where RTD is again the Real Time Difference between the KT 1 and the UT2. Geometric Time Difference for the SR 6, GTD_(SR) is given by theequation:GTD _(SR) =[d(SR,UT)−d(SR,KT)]/c   (4)where d(SR,UT) is the distance between the SR 6 and the UT 2, d(SR,KT)is the distance between the SR 6 and the KT 1 and c is the speed ofradio waves.

(iii) The measurements are then used to solve RTD from (3) and (4),giving:RTD=OTD _(SR) −GTD _(SR) =OTD _(SR) −[d(SR,UT)−d(SR,KT)]/c.   (5)

(iv) Inserting (5) and (2) into (1) givesOTD _(FR) =RTD+GTD _(FR) =OTD _(SR)−[d(SR,UT)−d(SR,KT)]/c+[d(FR,UT)−d(FR,KT)]/c.   (6)

(v) Rearranging (6) to transfer unknown variables to the left hand side,and multiplying the whole equation by c gives:d(SR,UT)−d(FR,UT)=d(SR,KT)−d(FR,KT)+c*[OTD _(SR) −OTD _(FR])  (7)

Thus the right hand side contains only known quantities because the OTDvalues are measured, and the coordinates of the KT 1 and the coordinatesof both the FR 4 and the SR 6 are known. The left hand side of thisequation defines a hyperbola, which is a contour in the XY-plane, whoseevery point has the property that the distance difference from it to theFR 4 and SR 6 is constant and equals the right hand side of the equation(7).

(vi) The measurement is repeated at least twice with at least one of thereceivers FR 4 and SR 6 in a different position. This is illustrated inFIG. 2, which shows for this embodiment the FR 4 and the SR 6 in twolocations additional to their locations of FIG. 1, the three locationsbeing indicated by letters a, b and c. Thus three hyperbolas in totalare obtained by the calculation method described above. The threehyperbolas are also shown in FIG. 2 and it can be seen that theycorrespond to the three sets of locations of the FR 4 and the SR 6. Inthis embodiment the three sets of measurements are taken using threedifferent known transmitters KT 1, however a single known transmittercould be used.

(vii) The three hyperbolas cross at substantially a single location, thepoint of substantial coincidence providing an estimate for the locationof the UT 2. This point is obtained using normal hyperbolic equationsolving techniques (in a similar manner to those used when workingdirectly with the E-OTD location method for mobile handsets).

It should be noted that the SR 6 could alternatively remain in the sameposition for all three sets of measurements, whilst only the FR 4 moves.

A further point to note is that, for each set of measurements, both OTDvalues (i.e. measured by FR 4 and SR 6) should be measured within ashort time window, otherwise RTD drifts need to be compensated for inorder to achieve an acceptable level of accuracy. The length of the timewindow depends on how large the RTD drift is. For example, in a GSMnetwork according to the standards, RTD drift should be between −0.1 and0.1 ppm. If the exact drift is not known and compensated for, in theworst case RTD drift generates ˜30 m error per second.

In contrast to mobile station location, the position of the UT 2 can beassumed to be fixed. This means that it is possible to collect manymeasurements (and consequently calculate many hyperbolas). In order toachieve optimum accuracy, tens or more hyperbolas would be used inpractice. On the other hand poor quality measurements could worsenaccuracy if a sufficient number of good quality hyperbolas have alreadybeen obtained. Thus in addition to the data described above, althoughnot essential, measurement quality figures could be attached to each OTDmeasurement and used to determine a weight (or quality figure) for eachhyperbola. Thus when the hyperbolic equations are finally solved toproduce a location estimate for the UT 2, the hyperbolas should beweighted using these weights.

Once a location of the UT 2 has been ascertained, the calculatedlocation can be compared to existing information in the databaseprovided by the network operator to see whether there are anydiscrepancies. As well as simply comparing the database and calculatedcoordinates, the calculated location can be used to check otherinformation such as base station identity and transmission channel.

Firstly the calculated location is compared with the locationcoordinates provided in the database. If the two values differ by morethan a certain acceptable threshold (for example 500 m) then it can beconcluded that either the location information in the database is wrongor that the identification information of the UT 2 is wrong andconsequently that measurements have been taken for a differenttransmitter other than that intended. This can be illustrated with thefollowing example:

Let us suppose that the database says that a GSM BTS with Cell Identity(CI) of 100 has a BCCH frequency of 10 and a BSIC of 4. The databasefurther says that the BTS is located at x=1000, y=1000. By contrast thecalculated location gives x=2000, y=2000 for the measured BTS. There aretwo possibilities. The first possibility is that the real location ofthe BTS with a CI=100 is x=2000, y=2000 and the database has incorrectlocation coordinates. The second possibility is that the BTS with a BCCHfrequency of 10 and a BSIC of 4 is actually the BTS with CI=101 and thisBTS is incorrectly identified in the database as CI=100. In this case,the database could be checked to see whether there is also a BTS withCI=101 at x=2000, y=2000 and if so, the measured BTS is simplyincorrectly identified in the database. Thus the embodiment can be usedto check the accuracy and consistency of various database information.

Once the correct information has been ascertained, it can subsequentlybe used to monitor the position of mobile telephones and the like usingthe network. Furthermore, the data could be used to dynamically update aNetwork Management System (NMS).

It will be understood that the measured signals are sent to a networkmanagement unit which also performs the above-detailed calculations.

When implemented in GSM, a mobile station (MS) would be told by theserving BTS to take measurements for certain neighbouring base stationsfor handover purposes. The MS can identify the base stations by theirBCCH frequency and BSIC value and can be told to measure base stationshaving certain values of BCCH frequency. The first and second receiverscould both be provided with the same list of base stations for which totake measurements, which in practice would be the same list as isprovided for handover.

Some examples of implementation of the embodiment are as follows:

1. One Mobile Telephone and One or More Location Measurement Units(LMUs)

This example is illustrated in FIG. 3. The one or more LMUs act as theSR 6 and a mobile telephone acts as the FR 4. The KT 1 and the UT 2 areboth base stations. Different hyperbolas are obtained by moving themobile telephone to different locations, whilst the LMU stays in oneposition.

2. Two Mobile Telephones

This example is illustrated in FIG. 4. One of the mobile telephones actsas the FR 4 and the other as the SR 6. The KT 1 and the UT 2 are bothbase stations. Both mobile telephones are moved to different locationsin order to obtain different hyperbolas. One requirement is that thereal position of the mobile telephones is known (e.g by GPS). Anotherrequirement is that both mobile telephones should be able to receivesimultaneously signals from the KT 1 and the UT 2.

3. One Mobile Telephone

This example is illustrated in FIG. 5. In this case the mobile stationacts as the first and the second receiver. In one position the OTDmeasurement is used as a measurement from the first receiver, and oncethe mobile has moved sufficiently, another measurement is used as ameasurement from the second receiver. It should be noted that thisexample works effectively only if the RTD drift between the basestations is slow.

Thus the embodiment allows the accuracy and consistency of networkinformation to be monitored and checked.

It can be understood by those skilled in the art that the invention iseasy to implement because routinely provided equipment is used. In itssimplest form, only a single, moveable receiver having E-OTDfunctionality and provided with a GPS is required. The calculations canbe performed by any suitable network management unit, such as a ServingMobile Location Center (SMLC). Thus a network operator could usesubscribers having mobile telephones equipped with E-OTD/GPS toautomatically monitor the network. Another possibility is just to dotest measurements with an E-OTD phone and separate GPS equipment.

It can also be appreciated that the method of the invention could beused to locate a moving mobile telephone using the same calculationprinciples. However, one particularly useful implementation is theability to locate accurately a base station during installation of alocation system without the need to physically visit the base station.This could provide a useful saving in labour costs.

The above-described embodiment can be easily implemented in networkssupporting E-OTD location method in GSM (2G), or OTDOA-IPDL method inUMTS (3G).

1. A telecommunications system comprising: a first transmitter unitsituated at a first, known location; a second transmitter unit situatedat a second, unknown location; a first receiving unit at a third, knownlocation arranged to receive signals from the first and secondtransmitter units; and a second receiving unit at a fourth, knownlocation arranged to receive signals from the first and secondtransmitter units, wherein the said signals received by the first andsecond receiving units are usable to ascertain the location of thesecond transmitter unit.
 2. A telecommunications system according toclaim 1, wherein the signals are indicative of the time taken for thesignals to arrive at the first and second receiving units from the firstand second transmitters.
 3. A telecommunications system according toclaim 2, wherein the signals are used to determine the time differencebetween the arrival times of signals at the first and second receivingunits from the first and second transmitters.
 4. A telecommunicationssystem according to claim 1, wherein the first and/or second receivingunits are moveable between a plurality of locations and are botharranged to receive a pair of signals when in each of the plurality oflocations, the said pair of signals comprising a signal from the firsttransmitter unit and a signal from the second transmitter unit.
 5. Atelecommunications system according to claim 4, wherein a said pair ofsignals received by the first receiving unit and a said pair of signalsreceived by the second receiving unit are together useable to calculatea range of possible locations of the second transmitter unit.
 6. Atelecommunications system according to claim 5, wherein the range ofpossible locations is in the form of a hyperbola in the X-Y plane inwhich the second transmitter unit is located, the said hyperbola runningthrough substantially the location of the second transmitter unit.
 7. Atelecommunications system according to claim 5, wherein in each of theplurality of locations the first and second receiving units receivepairs of signals which differ from those pairs of signals received whenthe first and second receiving units are in others of the plurality oflocations and the said different pairs of signals are together usable tocalculate different ranges of possible locations of the secondtransmitter unit.
 8. A telecommunications system according to claim 7,wherein the different ranges of possible locations substantiallycoincide at a single common location that is substantially the locationof the second transmitter unit.
 9. A telecommunications system accordingto claim 4, wherein, in any given location of the first and secondreceiving units, the pair of signals received by the first receivingunit is the same pair of signals that is received by the secondreceiving unit.
 10. A telecommunications system according to claim 4,wherein in any given location of the first and second receiving units,the pair of signals received by the first receiving unit is a differentpair of signals from the pair of signals received by the secondreceiving unit.
 11. A telecommunications system according to claim 4,wherein the plurality of locations is three locations.
 12. Atelecommunications system according to claim 1, wherein the signalsreceived by the first and second receiving units are received inresponse to signals sent to the first and second transmitter units bythe first and second receiving units.
 13. A telecommunications systemaccording to claim 2, wherein the said signals are further indicative oftheir quality or accuracy.
 14. A telecommunications system according toclaim 1, wherein the first and second receivers are separate entities.15. A telecommunications system according to claim 1, wherein the firstand second receivers are the same entity.
 16. A telecommunicationssystem according to claim 15, wherein the said same receiver entity isarranged to act as the said first receiver during a first period of timeand as the said second receiver during a second separate period of time.17. A telecommunications system according to claim 1, wherein one orboth of the first and second receivers is a mobile telephone.
 18. Atelecommunications system according to claim 17, wherein the said mobiletelephone supports Enhanced Observed Time Difference (E-OTD) locationmethod and Global Positioning System (GPS) location method, or ObservedTime Difference Of Arrival (OTDOA) location method and GlobalPositioning System (GPS) location method.
 19. A telecommunicationssystem according to claim 1, wherein one or both of the first and secondtransmitter units is a cellular base station.
 20. A telecommunicationssystem according to claim 1, wherein one or both of the first and secondreceivers is a location measurement unit.
 21. A telecommunicationssystem according to claim 1, wherein the second transmitter unit is in afixed location.
 22. A telecommunications system according to claim 1,further comprising a calculation unit arranged to use the signalsreceived by the first and second receiving units or any values derivedfrom the said signals to ascertain the location of the secondtransmitter unit.
 23. A telecommunications system according to claim 22,wherein the calculation unit is arranged to take account of theindication of quality or accuracy when using the signals received by thefirst and second receiving units.
 24. A telecommunications systemaccording to claim 22, located within a telecommunications network,wherein the calculation unit is a network management unit.
 25. Atelecommunications system according to claim 22, located within atelecommunications network, wherein the calculation unit is a ServingMobile Location Centre.
 26. A telecommunications system according toclaim 22, wherein the calculation unit is arranged to verify theaccuracy of the ascertained location of the second transmitter unit bycomparing it with location information of the second transmitter unitobtained from other sources.
 27. A telecommunications system accordingto claim 1, wherein the ascertained location of the second transmitterunit is usable to check the accuracy of identification information ofthe second transmitter unit obtained form other sources and thusidentify the second transmitter.
 28. A telecommunications systemcomprising: a first transmitter unit situated at a first, knownlocation; a second transmitter unit situated at a second, fixed, unknownlocation; a first receiving unit at a third, known location arranged toreceive signals from the first and second transmitter units; and asecond receiving unit at a fourth, known location arranged to receivesignals from the first and second transmitter units, wherein the saidsignals received by the first and second receiving units are usable toascertain the location of the second transmitter unit.
 29. Atelecommunications system comprising: a first base station situated at afirst, known location; a second base station situated at a second,unknown location; a first mobile station at a third, known locationarranged to receive signals from the first and second base stations; anda second mobile station at a fourth, known location arranged to receivesignals from the first and second base stations, wherein the saidsignals received by the first and second mobile stations are usable toascertain the location of the second base station.
 30. A method ofdetermining the location of a transmitter unit in a telecommunicationssystem, the method comprising the steps of: receiving signals at a firstreceiving unit situated at a first, known location from a firsttransmitter unit situated at a second, known location and from a secondtransmitter unit situated at a third, unknown location; receivingsignals at a second receiving unit situated at a fourth, known locationfrom the said first transmitter unit and from the said secondtransmitter unit; and using the received signals to ascertain thelocation of the second transmitter unit.
 31. A method of determining thelocation of a transmitter unit in a telecommunications system, themethod comprising the steps of: receiving signals at a first receivingunit situated at a first, known location from a first transmitter unitsituated at a second, known location and from a second transmitter unitsituated at a third, fixed, unknown location; receiving signals at asecond receiving unit situated at a fourth, known location from the saidfirst transmitter unit and from the said second transmitter unit; andusing the received signals to ascertain the location of the secondtransmitter unit.
 32. A method of determining the location of a basestation in a telecommunications system, the method comprising the stepsof: receiving signals at a first mobile station situated at a first,known location from a first base station situated at a second, knownlocation and from a second base station situated at a third, unknownlocation; receiving signals at a second mobile station situated at afourth, known location from the said first base station and from thesaid second base station; and using the received signals to ascertainthe location of the second base station.